In wireless mobile communication, when the distance between a base station (BS) and a wireless terminal such as a mobile station (MS) is long, a situation may occur in which a radio wave does not reach the base station from the wireless terminal while a radio wave reaches the wireless terminal from the base station. When such a situation occurs, it is difficult for the wireless terminal to request an access right or a transmission right from the base station.
For example, in WiMAX (IEEE802.16e) in which an Orthogonal Frequency Division Multiple Access (OFDMA) method is adopted, a wireless terminal transmits CDMA ranging used for requesting the assignment of a communication slot to a base station. Namely, when no communication slot is assigned to the wireless terminal, it is necessary for the wireless terminal to transmit CDMA ranging to the base station before transmitting data.
However, the number of subcarriers used for transmitting the CDMA ranging is large. For example, at the case of a 10 MHz bandwidth, 144 subcarriers are used for transmitting the CDMA ranging. On the other hand, in usual data transmission, the wireless terminal can transmit data with placing the data on 24 subcarriers when the number of subcarriers is a minimum value.
Here, in accordance with a standard or the like, a maximum value is set for the electric power of a wireless signal output from the wireless terminal. Therefore, when the number of subcarriers is increased, the amount of maximum power allocatable to each subcarrier is reduced. Namely, the amount of maximum power of each subcarrier used when the CDMA ranging is transmitted is small. Accordingly, even in an environment in which the base station can receive data if the data is transmitted from the wireless terminal to the base station using subcarriers the number of which is small, the CDMA ranging may not reach the base station in some cases. In this case, even though the wireless terminal can transmit and receive data to and from the base station, it is difficult for the wireless terminal to start or maintain communication with the base station. In addition, when data transmission from the wireless terminal is temporarily discontinued, the base station halts the assignment of a communication slot for the wireless terminal. When the data transmission is discontinued, it is necessary for the wireless terminal to transmit the CDMA ranging for data transmission. At this time, if no new CDMA ranging reaches the base station, the communication between the base station and the wireless terminal turns out to be disconnected. When the communication is disconnected, the base station determines the wireless terminal to be “out of service”.
The above-mentioned problem does not only relate to WiMAX (IEEE802.16e). Namely, the above-mentioned problem may occur in a wireless communication system in which data transmission is performed with data being placed on a plurality of subcarriers and a base station assigns an access right or a transmission right to a wireless terminal in response to a request from the wireless terminal.
As a related technique, a wireless communication method and a relay station have been proposed that effectively perform the unbiased assignment of a wireless resource for a subscriber station located outside the communication range of a base station. In this method, the relay station receives service request information from the subscriber station, and calculates a first necessary wireless resource necessary for a service indicated by the service request information, on the basis of the situation of communication with the subscriber station. In addition to this, on the basis of the situation of communication with the base station, the relay station calculates a second necessary wireless resource necessary for the service indicated by the service request information, and transmits, to the base station, service request information used for requesting the assignment of a third necessary wireless resource greater than or equal to the sum of the first necessary wireless resource and the second necessary wireless resource. An example of such a method is disclosed in Japanese Laid-open Patent Publication No. 2007-68092.